The invention relates to a retractable, hydraulic, telescopic shock absorber.
One such shock absorber includes a cylinder in which a piston, provided with passages is connected to a piston rod which can move through a cylinder cover and in which the piston divides the cylinder in two chambers. The cylinder chamber remote from the cylinder cover is by means of one or more passages in connection with a reservoir for the damping medium located outside the cylinder. In the other cylinder chamber is disposed an annular second piston freely slidable over the piston rod. Seals are provided between the second piston and the cylinder wall as well as the piston rod. The second piston has a rest position adjacent the cylinder cover. A pressure medium supplied from outside the shock absorber between the cylinder cover and the second piston serves to move the second piston in a direction remote from the cylinder cover.
Such a shock absorber is the subject of Dutch Patent 133 120 and is used for vehicles, such as fire, artillery and similar vehicles, in which it is necessary to obtain a stationary situation by suppression of the action of the springs of the vehicle such that a rigid connection is obtained between the coach-work of the vehicle and the wheels, as well as for amphibian vehicles for retracting the set of wheels to obtain a lower travelling resistance. The shock absorbers are typically arranged in parallel to the springs of the vehicle with the cylinder of the shock absorber fixed to the wheels and the piston rod connected to the chassis of the vehicle.
In shock absorbers without a second piston, the air present in the cylinder or liberated from the damping liquid is carried out through a passage with limited cross section between the cylinder and the reservoir in or near the cylinder cover. However, such a connection for venting is not allowable in shock absorbers having a second piston because the pressure medium supplied from outside the shock absorber between the cylinder cover and the second piston would escape to the reservoir with retraction of the shock absorber.